Interband cascade lasers with engineered carrier densities

ABSTRACT

Methods for improving the performance of type-II and type-I ICLs, particularly in the mid-IR wavelength range, are provided. The electron injector of a type-II or a type-I ICL can be heavily n-doped to increase the ratio of electrons to holes in the active quantum wells, thereby increasing the probability of radiative recombination in the active quantum wells and reducing the threshold current density J th  needed to achieve lasing. For both type-II and type-I ICLs, the doping should have a sheet density in the low-10 12  range. In either the type-II or the type-I case, in some embodiments, heavy doping can be concentrated in the middle quantum wells of the electron injector, while in other embodiments, doping with silicon can be shifted towards the active quantum wells.

CROSS-REFERENCE

This application is a Nonprovisional of and claims the benefit ofpriority under 35 U.S.C. §119 based on U.S. Provisional PatentApplication No. 61/477,191 filed Apr. 20, 2011 and U.S. ProvisionalPatent Application No. 61/596,870 filed on Feb. 9, 2012, both of whichare incorporated by reference into the present application in theirentirety.

TECHNICAL FIELD

The present invention relates to interband cascade lasers (ICLs),particularly improvement of the performance of ICLs by using doping inthe electron injector region to engineer the carrier densities in theactive quantum wells.

BACKGROUND

There has been an increasing interest in the development of lasersources that emit in the mid-infrared (“mid-IR”) spectral region,particularly at wavelengths between about 2.5 and 6 μm. Such lasers havesignificant uses for both military and non-military applications. In themilitary realm, mid-IR lasers can be extremely useful as acountermeasure to jam heat-seeking missiles and prevent them fromreaching their targets. In both the military and non-military realm,such mid-IR lasers have found use, for example, in chemical sensing, andso may be very useful in environmental, medical, and national securityapplications.

On the short-wavelength side of this spectral region, type-Iquantum-well antimonide lasers are achieving excellent performance andgreater maturity. See, e.g., T. Hosoda, G. Kipshidze, L. Shterengas andG. Belenky, “Diode lasers emitting near 3.44 μm in continuous-waveregime at 300K,” Electron. Lett. 46, 1455 (2010). On the long-wavelengthside of the mid-IR, intersubband quantum cascade lasers (QCLs) havebecome the dominant source of laser emissions. See, e.g., Q. Y. Lu, Y.Bai, N. Bandyopadhyay, S. Slivken, and M. Razeghi, “Room-temperaturecontinuous wave operation of distributed feedback quantum cascade laserswith watt-level power output, Appl. Phys. Lett. 97, 231119 (2010).

In recent years, the interband cascade laser (ICL) has been developed asanother promising semiconductor coherent source in the mid-IR range.

The first ICLs were developed by Dr. Rui Yang in 1994. See U.S. Pat. No.5,588,015 to Yang. The ICL may be viewed as a hybrid structure whichresembles a conventional diode laser in that photons are generated viathe radiative recombination of an electron and a hole. However, it alsoresembles a quantum cascade laser in that multiple stages are stacked asa staircase such that a single injected electron can produce anadditional photon at each step of the staircase. See S. Slivken, Y. Bai,S. B. Darvish, and M. Razeghi, “Powerful QCLs eye remote sensing,”Compound Semiconductor, pp. 22-23(2008); see also U.S. Pat. No.5,457,709 to Capasso et al.

Each stage of an ICL is made up of an active quantum well region, a holeinjector region, and an electron injector region. The photon cascade isaccomplished by applying a sufficient voltage to lower each successivestage of the cascade by at least one quantum of photon energy ℏω andallowing the electron to flow via an injector region into the next stageafter it emits a photon having that energy. See J. R. Meyer, I.Vurgaftman, R. Q. Yang and L. R. Ram-Mohan, “Type-II and type-Iinterband cascade lasers,” Electronics Letters, Vol. 32, No. 1 (1996),pp. 45-46 (“Meyer 1996”); and U.S. Pat. No. 5,799,026 to Meyer et al.,both of which are incorporated by reference into the present disclosure.

ICLs also employ interband active transitions just as conventionalsemiconductor lasers do. Each interband active transition requires thatelectrons occupying states in the valence band following the photonemission be reinjected into the conduction band at a boundary withsemi-metallic or near-semi-metallic overlap between the conduction andvalence bands. Most ICLs to date employ type-II active transitions wherethe electron and hole wavefunctions peak in adjacent electron (typicallyInAs) and hole (typically Ga(In)Sb) quantum wells, respectively, thoughICLs employing type-I transitions where the electron and holewavefunctions peak in the same quantum well layer have also beendeveloped. See U.S. Pat. No. 5,799,026 to Meyer et al., supra.

In order to increase the wavefunction overlap in type-II ICLs, two InAselectron wells often are placed on both sides of the Ga(In)Sb hole well,and create a so-called “W” structure. In addition, barriers (typicallyAl(In)Sb) having large conduction- and valence-band offsets can surroundthe “W” structure in order to provide good confinement of both carriertypes. See U.S. Pat. No. 5,793,787 to Meyer et al., which shares aninventor in common with the present invention and which is incorporatedby reference into the present disclosure in its entirety. Furtherimprovements to the basic ICL structure include using more than one holewell to form a hole injector. See U.S. Pat. No. 5,799,026 to Meyer etal., supra.

Additional early improvements to ICL design include those described inR. Q. Yang, J. D. Bruno, J. L. Bradshaw, J. T. Pham and D. E. Wortman,“High-power interband cascade lasers with quantum efficiency >450%,”Electron. Lett. 35, 1254 (1999); R. Q. Yang, J L. Bradshaw, J. D. Bruno,J. T. Pham, and D. E. Wortman, “Mid-Infrared Type-II Interband CascadeLasers,” IEEE J. Quant. Electron. 38, 559 (2002); and in K. Mansour, Y.Qiu, C. J. Hill, A. Soibel and R. Q. Yang, “Mid-infrared interbandcascade lasers at thermoelectric cooler temperatures,” Electron. Lett.42, 1034 (2006).

However, the performance of the first ICLs fell far short of thetheoretical expectations. In particular, the threshold current densitiesat elevated temperatures were quite high (5-10 kA/cm² at roomtemperature in pulsed mode) and fell only gradually to 1-2 kA/cm² for arelatively large number of stages, which precluded room-temperaturecontinuous-wave (cw) operation of those devices.

More recently, researchers at the U.S. Naval Research Laboratory (NRL)formulated and tested certain design changes tuning the configuration ofthe hole injector region within a given stage, the active quantum wellswithin a given stage, the electron injector region within a given stage,the active gain region comprising multiple stages, and/or the separateconfinement region. These design changes have dramatically improved ICLperformance, with the threshold current density falling to approximately400 A/cm² and the threshold power density to approximately 900-1000W/cm². See U.S. Pat. No. 8,125,706 Vurgaftman et al.; U.S. patentapplication Ser. No. 13/023,656 Vurgaftman et al. filed Feb. 9, 2011;and U.S. patent application Ser. No. 13/353,770 Vurgaftman et al. filedJan. 19, 2012, all of which share at least one inventor in common withthe present invention and are hereby incorporated by reference into thepresent disclosure in their entirety. As a consequence, in early 2010,an NRL device reached a maximum cw operating temperature of 72° C.,which was 60° C. higher than for any ICL designed elsewhere. NRL'sresearch on further improvements to ICL performance has continued.

SUMMARY

This summary is intended to introduce, in simplified form, a selectionof concepts that are further described in the Detailed Description. Thissummary is not intended to identify key or essential features of theclaimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter. Instead, it ismerely presented as a brief overview of the subject matter described andclaimed herein.

The present invention provides methods for improving the performance oftype-II and type-I ICLs, particularly in the mid-IR wavelength range.

In accordance with some aspects of the present invention, the electroninjector of a type-II ICL can be heavily n-doped to increase the ratioof electrons to holes in the active quantum wells, thereby increasingthe probability of radiative recombination in the active quantum wellsand reducing the threshold current density J_(th) needed to achievelasing. The doping sheet density should be in the range of about 1.5 toabout 7×10¹² cm⁻², with the best results in some cases being achievedwith doping in the range of about 1.5 to 2.5×10¹² cm⁻².

In accordance with other aspects of the present invention, the electroninjector of a type-I ICL also can be heavily n-doped to roughly equalizethe electron and hole populations in the active quantum wells. Fortype-I ICLs, the doping should have a sheet density of about 2 to about7×10¹² cm⁻², though in some cases doping in the range of about 2 to3×10¹² cm⁻¹ can achieve the best results.

In both the type-II and type-I cases, type-I the number of doped QWs inthe electron injector can be varied, and the doping can be concentratedin either the middle QWs of the electron injector or can be shiftedtowards the active quantum well of the next stage to achieve the bestutilization of the doping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the calculated band diagrams and probability densityprofiles for selected electron and hole subbands for one stage of atype-II interband cascade laser.

FIG. 2 illustrates the corresponding electron (n) and hole (p) chargedensities as a function of position within a conventional interbandcascade laser stage from the prior art, and having the layeringconfiguration illustrated in FIG. 1.

FIG. 3 illustrates the electron (n) and hole (p) charge densities as afunction of position for a type-II interband cascade laser in accordancewith a first embodiment of the present invention.

FIG. 4 is a plot of measured threshold current densities as a functionof Si doping level in the injector quantum wells of three exemplarytype-II interband cascade lasers from the prior art with moderate dopinglevels (the lowest in the figure) and five type-II interband cascadelasers having much higher doping in accordance with the presentinvention.

FIG. 5 is a plot illustrating the measured threshold power densities ofinterband cascade lasers in accordance with the present invention and ofinterband cascade lasers in accordance with the prior art.

FIG. 6 illustrates the measured internal losses of interband cascadelasers in accordance with the present invention and of interband cascadelasers in accordance with the prior art.

FIG. 7 shows the calculated band diagrams and probability densities forselected electron and hole subbands for one stage of a type-I interbandcascade laser in accordance with another embodiment of the presentinvention.

DETAILED DESCRIPTION

The aspects and features of the present invention summarized above canbe embodied in various forms. The following description shows, by way ofillustration, combinations and configurations in which the aspects andfeatures can be put into practice. It is understood that the describedaspects, features, and/or embodiments are merely examples, and that oneskilled in the art may utilize other aspects, features, and/orembodiments or make structural and functional modifications withoutdeparting from the scope of the present disclosure.

Until now, the process used in designing ICLs has taken for granted thatthe electron and hole densities in the active quantum wells areapproximately balanced at threshold. The inventors of the presentinvention have developed a detailed model of ICL operation that combinedk·p calculations of the band structure and optical gain with statisticalanalysis of the spatial profiles of the electron and holeconcentrations, in part in order to check this assumption. Based on theresults of these models, the inventors have discovered that in priorstate-of-the-art designs the threshold hole density is much larger thanthe corresponding electron density. This occurs because despite n-dopingof each stage in the conventional designs, most of the resultingelectrons reside in the electron injector rather than active QWs whereasa much larger fraction of the holes populates the active quantum wells.This has been true of all known ICLs fabricated or designed before thisinvention.

The present invention improves the performance of type-II and type-Iinterband cascade lasers (ICLs) by increasing the relative density ofelectrons in the active quantum wells, thus increasing the gain achievedfor a given injected current density and reducing the rate ofnon-radiative recombination due to Auger processes. It will be noted atthis point that the term “ICL” is used herein to refer to both aninterband cascade gain medium and a laser incorporating an interbandcascade gain medium, and both such a gain medium and such a laser arecontemplated to be within the scope and spirit of the presentdisclosure.

In both the type-II and type-I cases, this improved performance can beaccomplished by increasing the n-type doping density in the electroninjector by about an order of magnitude, which has the effect ofradically altering the balance between the electron and hole densitiesin the active quantum wells of the device when it reaches threshold.Calculations of optical gain by the inventors of the present inventionconfirm that up to a certain point, increasing the electron-to-holedensity ratio at threshold should also significantly reduce thethreshold current density. This basic idea of the invention wasdramatically verified when ICLs grown by molecular beam epitaxy (MBE) tothe new design consistently displayed substantially lower thresholds andhigher efficiencies.

Thus, in accordance with some aspects of the present invention,performance of a type-II ICL can be improved by greatly increasing then-doping of the electron injector region to substantially enhance theelectron population in the active quantum wells. As described in moredetail below, in some embodiments, the heavy doping can be concentratedin the middle quantum wells of the electron injector, while in otherembodiments, the doping can be shifted towards the end of the electroninjector closer to the active quantum wells of the next stage (e.g.toward the right as shown in FIG. 3). The doping sheet density should bein the range of about 1.5 to about 7×10¹² cm⁻² with the best results insome cases being achieved with doping in the range of about 1.5 to2.5×10¹² cm⁻².

FIG. 1 shows the calculated band diagrams and electron and holewavefunctions for selected electron and hole subbands in one stage of anexemplary modeled type-II ICL designed to emit at λ=3.7 μm. FIG. 2 is aplot of the corresponding electron and hole charge densities n and p asa function of their respective position within an ICL having thestructure shown in FIG. 1 and having doping levels in accordance withthe prior art, while FIG. 3 illustrates aspects of electron and holedensities for a type-II ICL having doping levels in accordance with thepresent invention.

Thus, FIG. 1 shows elements of an ICL, including active QWs 101 a andhole injector 102 consisting of one or more hole quantum wells for afirst ICL stage and electron injector 103 and active QWs 101 b for asecond ICL stage. In accordance with principles of ICL operation knownin the art, electrons from an electron injector (not shown but havingthe same structure as electron injector 103) and holes from holeinjector 102 are injected into active QWs 101 a. The injected electronsand holes recombine in active QWs 101 a by releasing photons. Theelectrons then tunnel through the hole injector region 102 and intoelectron injector 103 and are injected into active QWs 101 b of the nextstage to be recombined with holes from a corresponding hole injector.See Meyer 1996 and U.S. Pat. No. 5,799,026 to Meyer et al., supra.

It can be seen from FIG. 1 that the electron injector states nearest thehole injector (e.g., wavefunction 105 in electron injector 103) havelower energies than the electron states in the next active QWs 101 b, asshown by electron wavefunctions 106. As a consequence, most of theinjected electrons remain in electron injector region 103 rather thantransferring to the active quantum wells 101 b where they can beavailable to recombine with the holes injected into that stage. On theother hand, nearly all of the injected holes transfer to the active holequantum well while a very small fraction remains in the hole injector102. The result is an imbalance of electrons and holes in the activeQWs, which contributes to a higher rate of Auger recombination and aneed for a higher threshold current density to achieve lasing.

The plots shown in FIG. 2 illustrate this phenomenon. FIG. 2 depicts anICL in accordance with the prior art, in which the electron injector ismoderately doped. Thus, as shown in FIG. 2, four of the electroninjector's InAs QWs, which have thicknesses of 42, 32, 25, and 20 Å,respectively, are n-doped to have donor densities of 4×10¹⁷ cm³, with atotal sheet doping density of 4.8×10¹¹ cm⁻². This sheet doping densityis lower than the threshold carrier density at room temperature,although the semimetallic interface 107 between the electron and holeinjectors can generate additional charge-balanced carrier densities (thesame for electrons and holes) with increasing bias voltage. Thesefield-generated carrier densities depend on the thicknesses of theindividual wells in the electron injector as well as the electric field,and can be on the order of 10¹² cm⁻².

As shown by the plots in FIG. 2, the ratio of electron and hole sheetdensities N_(s)/P_(s) is much less than unity in the active quantumwells. Thus, although charge balance implies that the total electronsheet density must exceed the total hole sheet density, FIG. 2 indicatesthat a large number of excess electrons reside in the electron injectorrather than active quantum wells, and hence do not contribute to thegain. This is confirmed by the relative peak in the n plot of electroncharge density in the electron injector as compared to the values of nin the active quantum wells.

The origin of this effect is understood as follows. In order to makegood use of the applied voltage, it is desired that the voltage drop perstage of an ICL be close to the photon energy ho (in practice, thepresence of cavity losses makes the voltage drop larger than ho byapproximately 10-15 meV). Therefore, as illustrated in FIG. 1, the topof the heavy-hole subband in the active quantum wells 101 a of one stageis closely matched to the bottom of the active electron subband 106 inthe next stage. In order to depopulate most of the electrons from theinjector and cause them to move to the active quantum wells, it would benecessary to raise the electron subbands in the electron injector by afew kT.

However, this is inadvisable because the only holes in the device comefrom the semimetallic interface 107 between the hole injector and theelectron injector, and the energy overlap (the difference between thetop of the hole subband in the active hole quantum well and the lowestelectron energies in the electron injector) at that interface should bemaintained at approximately 30 meV to ensure that holes will populatethe active quantum wells. If the energy overlap were to vanish,interband radiative transitions would be much weaker because too manyholes would populate the hole injector and thus too few would populatethe active hole quantum well. However, while this energy overlap isbeneficial for hole movement in the active quantum wells as noted above,because the electron injector states are lower in energy and thus morelikely to be occupied than the active electron states, most of theelectrons reside in the injector rather than the active quantum wellsand thus are not available to recombine with the holes and participatein the gain.

As described in more detail below, the improvements to ICL design inaccordance with the present invention provide a better balance betweenthe electron and hole densities in the active quantum wells so thatradiative recombination and therefore lasing can be achieved with lowerthreshold currents and lower losses for higher efficiencies.

The optimum balance between the electron and hole sheet densities N_(s)and P_(s), respectively in the active quantum wells depends on suchfactors as the dominant recombination mechanism, the relative freecarrier absorption cross sections, the required threshold gain, and therelative densities of states in the conduction and valence bands. Atthermoelectric-cooler temperatures in excess of 250 K, which arerequired for practical applications of the mid-IR lasers, nonradiativeAuger recombination is known to strongly dominate all otherrecombination processes. A simple model for the threshold currentdensity limited by Auger recombination has the following form:J _(th) =qγ ₃ [rN _(s) ² P _(s)+(1−r)N _(s) P _(s) ²]/(d ²η_(i))where γ₃ is net Auger coefficient if N_(s)=P_(s), r is the fraction ofAuger recombination that arises from multielectron processes, η_(i) isthe internal efficiency, which is known from cavity-length measurementson ICLs to range from 70 to 80%, and d is the normalization lengthrequired if the Auger coefficient is expressed in the standard 3D units.

When Auger recombination dominates, the optimum electron and holedensities are determined by the relative Auger rates for multi-electronvs. multi-hole processes. If the rate is much faster for multi-holeprocesses (r→0), it is preferable to have many more electrons thanholes, whereas for multi-electron processes (r→1), it is preferable tohave more holes.

Apart from the considerations described above, in real devices theinternal loss may also depend on the relative carrier densities,possibly in a complicated way, because the free carrier absorption crosssections are different for electrons and holes. The required opticalgain at room temperature may be estimated from measurements of theinternal loss and the measured threshold current density in pulsed mode.

The inventors of the present invention have previously found thatraising some of the higher-lying subbands in the electron injector withrespect to the active subband can reduce the number of electronsresiding in the injector. See U.S. patent application Ser. No.13/023,656 to Vurgaftman et al., supra. However, this is insufficient toensure that an adequate number of injected electrons are transferred tothe active quantum wells because some injector states still are lower inenergy than the active states and electrons tend to remain in theseinjector states.

Thus, to solve these problems, in accordance with some aspects of thepresent invention, the electron injector in a type-II ICL can be heavilyn-doped far beyond any level employed previously, so that even if mostof the total electron population remains in the injector, the electrondensity nonetheless exceeds the hole density in the active QWs. In apreferred embodiment, the dopant is silicon, and the sheet dopingdensity is between about 1.5 and 2.5×10¹² cm⁻², though of course otherdopants such as Te and/or somewhat higher doping concentrations up to7×10¹² cm⁻² may be used as appropriate.

FIG. 3 illustrates an exemplary embodiment of a heavily n-doped ICLstructure in accordance with the present invention. As in the ICLillustrated in FIG. 2, the electron injector of the ICL illustrated inFIG. 3 includes InAs electron quantum wells having thicknesses of 32,25, 20, and 17 Å, respectively. In accordance with the presentinvention, these electron quantum wells are heavily n-doped, for a totalsheet doping density of about 4.7×10¹² cm⁻². In the embodimentillustrated in FIG. 3, as in the prior art ICL illustrated in FIG. 2,the n-doping is concentrated in the four QWs in the middle of theelectron injector region.

Like FIG. 2, FIG. 3 contains plots of the electron (n) and hole (p)charge densities at different positions within the ICL structure. Acomparison of the n and p plots shown in FIG. 3 with the correspondingplots for a prior art ICL shown in FIG. 2 clearly shows the benefits ofthe heavy doping of the electron injector in accordance with the presentinvention. In the ICL shown in FIG. 2, the hole sheet density P_(s)substantially exceeds the electron sheet density N_(s) in the activeQWs, while in the ICL shown in FIG. 3, the electron sheet density N_(s)is slightly larger than P_(s) in the active QWs as a result of the heavydoping, indicating a more favorable balance between electrons and holes.

Thus, as will be readily appreciated by ones skilled in the art, inaccordance with the present invention, the electron and hole densitiesin the active quantum wells can be optimized by tuning the dopingdensity of the electron injector.

To test the effects of the heavy n-doping of the electron injector QWsin accordance with the present invention, a series of samples withdifferent doping densities was grown, processed, and characterized,yielding the experimental results summarized in Table 1 shown below.

TABLE 1 Doping Sheet N_(s) P_(s) γ₃ Sam- Density λ (10¹¹ (10¹¹ α_(i)J_(th) (10⁻²⁷ ple (cm⁻²) (μm) cm⁻²) cm⁻²) (cm⁻¹) (A/cm²) cm⁶/s) 1 4.8 ×10¹¹ 3.76 3.29 17.2 12.2 378 3.16 2 4.8 × 10¹¹ 3.77 3.35 17.3 13.1 3883.15 3 4.8 × 10¹¹ 3.92 3.51 17.6 15.6 421 3.16 4 2.4 × 10¹² 3.90 4.5910.8 8.5 247 3.12 5 4.7 × 10¹² 3.78 7.23 6.10 7.9 188 3.02 6 4.7 × 10¹²3.67 7.10 5.98 6.5 167 2.84 7 6.6 × 10¹² 3.84 10.0 4.14 9.0 211 3.36 87.4 × 10¹² 3.82 15.3 2.69 11.2 258 3.19

In all cases studied to produce Table 1, the ICL had a structure such asthat illustrated in FIGS. 1, 2, and 3, with an electron injectorcomprising a plurality of InAs QWs alternating with AlSb barriers.Samples 1-3 were grown using a conventional ICL design with the moderateinjector doping of the prior art. In Samples 4-7, the four InAs QWs inthe middle of the electron injector were doped as illustrated in FIG. 3.In Sample 8, the final and most heavily doped sample, only the last twoInAs QWs of the electron injector were doped in order to moreefficiently transfer electrons into the active quantum wells.

As described in more detail below, in accordance with the presentinvention, the threshold electron (N_(sth)) and hole (P_(sth)) densitiesin the active quantum wells can be achieved by tuning the doping densityof the electron injector. The threshold electron and hole sheetdensities for each of these designs were calculated at the thresholdgain, which is equal to the sum of the measured internal loss α_(i) anda mirror loss of 4.5 cm⁻¹ for the 2 mm cavities with uncoated facets.The optical gain was calculated using the proper optical confinementfactor of the ICL core. While the emission wavelengths of these devicesspanned from 3.67 to 4.01 μm, these and earlier devices have displayedweak dependence on wavelength in this range so for simplicity we assumedthe same wavelength for all of them.

Using the calculated electron and hole densities at threshold shown inTable 1, the inventors performed a two-parameter fit (r, γ₃) to theobserved threshold carrier densities using the expression for theAuger-limited threshold current density given above and converting the2D Auger coefficient into its 3D form γ₃ (for a straightforwardcomparison with bulk materials) using a nominal normalization length ofd=100 Å along the growth direction. The best fit was obtained usingr=0.51 (which implies near equality between multi-electron andmulti-hole Auger processes) and γ₃=3.1×10⁻²⁷ cm⁶/s. The inventors alsofound that they could fix r at this fit value to estimate the spread inthe Auger coefficient, which results in the values for each sample givenin the final column of Table 1.

As shown in Table 1 above, both the room-temperature threshold carrierdensity and internal loss are reduced by the introduction of thepreferred implementation of this invention. The reduction is by over afactor of 2 in the case of J_(th) and nearly a factor of 2 in the caseof α_(i).

From the results shown in Table 1, the inventors deduced that in theoptimal scenario, where the threshold current densities J_(th), internallosses α_(i), and net Auger coefficient γ₃ are minimized, was achievedin Samples 5 and 6 shown in Table 1. Thus, the most favorable propertieswere obtained when the electron injector had a net sheet doping densityof 4.7×10¹² cm⁻². For that doping level, the threshold electron sheetdensity N_(sth) in the active quantum wells is about 7×10¹¹ cm⁻², whilethe threshold hole sheet density P_(sth) in the active quantum wells isabout 6×10¹¹ cm⁻¹. However, somewhat higher sheet doping densities areneeded when the internal loss increases significantly, as has beenobserved for wavelengths longer than 4 μm.

In some embodiments in accordance with the present invention, the dopingis concentrated in the middle QWs of the electron injector. In otherembodiments, the doping can be shifted towards the active quantum wells,for example to include doping of the injector quantum well directlyadjacent to the active quantum wells or with one undoped injectorquantum well separating the doped quantum wells from the active quantumwells as shown in FIG. 3, which causes a larger fraction of the injectedelectrons to populate the active quantum wells, though it may beexpected that lasing may suffer due to non-radiative recombination atdefects if the active quantum wells themselves or possibly the wellimmediately adjacent to them were heavily n-doped. In other embodiments,both the wells and the bathers can be doped to obtain the specifiedsheet doping density.

In a preferred embodiment, the injector can be designed so that thedevice can operate with the lowest amount of doping that is required tobring the electron/hole population ratio in the active quantum wells toan optimal value roughly on the order of 1.3 but extending from about0.7 to about 2. The minimum amount of doping is expected to result inreduced internal loss for the devices. However, the inventors have notyet explored the limits of operability in these circumstances, so theseguidelines are based on simulations in which the electron subband of thefollowing stage is 15-20 meV above the ground subband of the electroninjector. Future experimental work will allow the inventors to betterpinpoint the minimum separation needed to assure good electron transportthrough the injector into the following stage.

The optimum injector design changes as a function of wavelength.Although in the embodiments described herein, the ICLs were configuredto operate in the 3-6 μm spectral region, one skilled in the art wouldreadily appreciate that the doping principles in accordance with thepresent invention can be straightforwardly extended to ICLs operating atshorter or longer wavelengths. At all wavelengths, the following designrules hold in the preferred embodiment: (1) the thickness of theinjector wells is gradually reduced from the semimetallic interface tothe active quantum wells of the following stage; (2) the thickness ofthe two InAs injector wells near the active quantum wells is 0-2 Ålarger than that of the first active InAs well; (3) the InAs wells inthe injector are separated by about 12 Å AlSb barriers.

In an exemplary embodiment, the thickness of the InAs injector well nextto the semimetallic interface varies from about 30 Å for λ=3 μm with 6injector wells to about 35 Å for λ=6 μm with 5 injector wells, with atotal injector thickness of 180-200 Å. Configurations with slightlydifferent numbers of wells are also possible. The optimum injectordoping is given by a total sheet doping density that varies from about1.8×10¹² cm⁻² for emission at λ=3 μm to about 2.3×10¹² cm⁻² for emissionat λ=6 μm, although these doping densities can vary by 20-30% withoutsignificant degradation in the laser performance.

In some embodiments, such optimum doping can be realized by dopingequally the 4 or 3 wells in the center of the injector. However,configurations in which fewer or more wells are doped are also possible,as long as the total sheet density is kept in the range of about 1.5 toabout 2.5×10¹² cm⁻². For example, for operation at λ=3.7 μm, an electroninjector for an ICL in accordance with the present invention can havethe following structure: 25 Å AlSb (at the semimetallic interface, notcounted as part of the injector)/30 Å InAs/12 Å AlSb/27 Å InAs/12 ÅAlSb/24 Å InAs/12 Å AlSb/20 Å InAs/12 Å AlSb/18 Å InAs/12 Å AlSb/18 ÅInAs/25 Å AlSb (exit barrier, not counted as part of the injector) withthe 27, 24, 20, and 18 Å wells n-doped to 2×10¹⁸ cm³ for a total sheetdoping density of 1.8×10¹² cm⁻².

In other embodiments, an ICL having engineered carrier densities inaccordance with the present invention can include designs with somewhatthicker injector regions in which the doping is somewhat heavier. Suchdesigns with thicker injector regions and heavier doping were employedin the experimental study described above and associated with Table 1,which resulted in quite favorable laser performance.

In one such embodiment, the thickness of the injector well next to thesemimetallic interface is approximately 42 Å at all wavelengths. Foremission at λ=3 μm, the total injector thickness should be about 220 Åand the optimum sheet doping level is about 5×10¹² cm⁻², while at anemission wavelength λ=6 μm, the injector should have a thickness ofabout 180-190 Å and an optimum sheet doping level of about 3×10¹² cm⁻².In these embodiments as in the previously described embodiments havinglower doping levels, the best results were achieved by doping three orfour wells, shifted towards the active quantum wells.

FIG. 4 plots the measured threshold current densities J_(th) as afunction of the sheet Si doping level in the injector QWs. The plot inFIG. 4 shows a clear experimental minimum in J_(th) for a sheet dopingdensity of about 5×10¹² cm⁻². The theoretical minimum is somewhat wider,spanning sheet densities between 5 and 7×10¹² cm⁻². Although part of theJ_(th) reduction with optimized doping arises from the concomitantdecrease of the internal loss, most of it is due to the more effectivebalance between electron and hole populations described above. Since theloss is found to be sensitive to injector doping level, one implicationis that a substantial fraction of the loss in ICLs with conventionaldesigns from the prior art (without heavily-doped electron injectors)originated in the active core of the devices. Based on theseexperimental and modeling results, the preferred embodiment of thepresent invention has a sheet n-doping density in the 1.5-7×10¹² cm⁻²range, with one to four wells in the middle of the injector doped withSi to achieve this sheet density range.

Thus, using their recent detailed band structure and statisticalmodeling, the inventors have found that the layering and dopingconfigurations employed in all known previous ICL designs resulted inhole densities in the active quantum wells that far exceeded thecorresponding electron densities at threshold.

However, the simulations further showed that this unfavorablerelationship can be corrected by very heavily doping some of the QWs inthe electron injector. By heavily doping the electron injector inaccordance with the present invention, electron density in the activequantum wells can be made slightly larger than the hole density atthreshold. Subsequent growths of ICL wafers employing the invention haveconfirmed the theoretical predictions, in that the laser performancecharacteristics measured for the new structures following the inventionare far superior to any obtained previously using designs from the priorart.

The improvements to ICL performance resulting from n-doping the electroninjector in accordance with the present invention include higheroperating temperatures, higher output powers, higher wall-plugefficiencies, and lower power consumption requirements when narrowridges are processed from the material. Preliminary experimental studiesof structures grown according to the invention have demonstratedsubstantial performance improvements over all previous interband cascadelasers.

The plots in FIGS. 5 and 6 further illustrate some of the advantages ofthe present invention.

FIG. 5 is a plot of threshold power density P_(th) for wavelengths inthe mid-IR range of 3.0 to 5.6 μm for lasers having the heavily-dopedelectron injectors described above in accordance with the presentinvention as compared to lasers produced according to the prior art andaccording to the inventors' previous improvements on the prior art asdescribed in U.S. Pat. No. 8,125,706 and U.S. patent application Ser.No. 13/023,656. FIG. 5 clearly shows that all of the lasers grownaccording to the invention display a dramatic reduction of the thresholdpower density for all wavelengths as compared to any of the prior artdesigns.

FIG. 6 similarly plots internal loss for wavelengths in the same mid-IRrange. Although the differences are not as stark as for the thresholdpower density shown in FIG. 5, FIG. 6 nevertheless shows that internallosses are generally less for lasers designed in accordance with thepresent invention than for lasers designed in accordance with the priorart.

Thus, rebalancing the electron/hole population ratio in the activequantum wells via heavy doping of the electron injector in accordancewith the present invention improves all of the major laser performancefigures of merit rather than just the threshold current density.

The present invention also provides a set of design changes to optimizethe relative proportion of electrons and holes in the active quantumwell of a type-I ICL so that lasing can be achieved with reducedthreshold current density J_(th) and reduced internal losses.

As described in more detail below, as with the embodiments describedabove with respect to type-II ICLS, in accordance with the presentinvention, the electron injector of a type-I ICL can also be heavilyn-doped to achieve a desired ratio of electrons and holes in the activequantum well, with the same beneficial results. For type-I ICLs, then-type doping with silicon, Te, or some other suitable dopant shouldhave a sheet density of about 2 to about 7×10¹² cm⁻², though in somecases doping in the range of about 2 to about 3×10¹² cm⁻² will achievethe best results.

These aspects of the present invention employ type-I active QWs ratherthan the type-II InAs/Ga(In)Sb active gain layers that have been used inall high-performance ICLs from the prior art. Type-I ICLs with electronsand holes localized in the same layer are described in U.S. Pat. No.5,799,026 to Meyer et al. and Meyer 1996 supra. A related type-I ICL wasalso demonstrated experimentally. See S. R. Kurtz, A. A. Allerman, R. M.Biefeld, and K. C. Baucom, “High slope efficiency, ‘cascaded’midinfrared lasers with type I InAsSb quantum wells,” Appl. Phys. Lett.72, 2093 (1998). However, that demonstration yielded quite poorperformance compared to the more recent state of the art, and ifimplemented the theoretical designs from that period would similarlyhave failed to compete favorably with current devices.

FIG. 7 illustrates an example of a preferred embodiment of a type-I ICLin accordance with the present invention, designed for room-temperatureemission at 3.4 μm. This embodiment of a type-I ICL substitutes anInGaAsSb active QW for a type-II InAs/GaInSb/InAs active QWs of thecurrent state of the art for type-II ICLs according to the teachings ofother recent NRL Patent Applications. The active QW is similar to thatdescribed in the recent work on type-I multiquantum-well mid-IR diodelasers, such as Hosoda et al., supra; T. Lehnhardt, A. Herrmann, M.Kamp, S. Höfling, L. Worschech, and A. Forchel, “Influence of GaSb andAlGaInAsSb as Barrier Material on ˜2.8 μm GaSb-Based Diode LaserProperties,” IEEE Phot. Tech. Lett. 23, 371 (2011); L. Naehle, S.Belahsene, M. von Edlinger, M. Fischer, G. Boissier, P. Grech, G. Narcy,A. Vicet, Y. Rouillard, J. Koeth and L. Worschech, “Continuous-waveoperation of type-I quantum well DFB laser diodes emitting in 3.4 mmwavelength range around room temperature,” Electron. Lett. 47, 46(2011); S. Belahsene, L. Naehle, M. Fischer, J. Koeth, G. Boissier, P.Grech, G. Narcy, A. Vicet, and Y. Rouillard, “Laser Diodes for GasSensing Emitting at 3.06 μm at Room Temperature,” IEEE Phot. Tech. Lett.22, 1084 (2010); and G. Kipshidze, T. Hosoda, W. L. Sarney, L.Shterengas, and G. Belenky, “High-Power 2.2-μm Diode Lasers withMetamorphic Arsenic-Free Heterostructures,” IEEE Phot. Tech. Lett. 23,317 (2011), all of which are incorporated by reference into the presentapplication.

Apart from the active QW, this embodiment maintains most of the featuresof the prior art for type-II ICLs. The gain is generated by a 100 ÅIn_(0.6)Ga_(0.4)As_(0.3)Sb_(0.7) QW with 1.6% compressive strain, andthe emission wavelength can be straightforwardly extended throughout therange 3.0 to 4.2 μm by adjusting the InGaAsSb QW composition andthickness and making other appropriate adjustments to the injectorregions. For example, in order to extend the operation to longerwavelengths, the In and As fractions in the active QW should beincreased (while maintaining roughly the same strain level), in tandemwith the thickness of the injector wells. Even longer wavelengths arepossible if the AlSb barriers are replaced by strained InAlAs barriers.

In the embodiment illustrated in FIG. 7, 25 Å AlSb barriers separate theactive QW from the electron injector on one side and the hole injectorconsisting of one or more hole quantum wells on the other side. The holeinjector in this embodiment consists of 30 and 45 Å Al_(0.2)Ga_(0.8)SbQWs, the thicknesses of which are adjusted so that they are about 70 meVbelow the top of the active hole subband in the InGaAsSb QW. In otherembodiments, GaAsSb or AlGaAsSb hole wells with similar thicknesses canbe used. The two wells of the hole injector are separated by a 10 Å AlSbbarrier. As in the type-II ICLs discussed above, the electrons and holesthat produce gain are generated at the semimetallic interface betweenthe electron and hole injectors, which are again separated by a 25 ÅAlSb barrier.

The electron injector is comprised of five chirped InAs/AlSb QWs, byanalogy with the inventors' previous patent on ICLs, U.S. Pat. No.8,125,706, incorporated by reference into the present application in itsentirety. In an exemplary embodiment, the first InAs QW in the electroninjector has a thickness of 45 Å and the last QW has a thickness of 25Å. Although the positions of the valence-band maximum andconduction-band minimum in InGaAsSb are somewhat uncertain (while theuncertainty in the energy gap is considerably smaller), the InAs QWs inthe electron injector should be designed so that the lowest subband ofthe injector is 15-20 meV below the active subband and the highestsubband of the injector is 120-170 meV above the active subband. Weestimate that the thicknesses may need to vary between about 35 and 55 Åfor the first QW and about 20 and 30 Å for the last QW, with the otherQW thicknesses falling between those values, in order to satisfy thisrequirement. This further implies that the thickness of the QWs willchange with the desired emission wavelength, as described in theprevious ICL patents. 12 Å AlSb separate all the QWs of the electroninjector.

Thus, in an exemplary embodiment in accordance with these aspects of thepresent invention, the three middle QWs of a type-I ICL, withthicknesses of 36, 30, and 26 Å, respectively, are doped to about2.5×10¹⁸ cm⁻³ for a total sheet doping density of about 2.3×10¹² cm⁻².As with the type-II design, the number of doped QWs in a type-I ICL canbe varied, and as in the type-II design the best utilization of thedoping level is achieved when the doping is shifted as close to theactive QW of the next stage as can be done without introducing excessivedefects that adversely affect the non-radiative lifetime or other activeQW properties.

The type-I ICL described here is expected to combine the advantages ofrecent non-cascade type-I diode lasers incorporating InGaAsSb active QWssuch as that described in Hosoda et al., supra, with the lower thresholdcurrent densities and ohmic losses of a cascade geometry. It will alsoavoid the need to calibrate and grow quinternary InGaAlAsSb barriersbetween the QWs that are required to maximize the hole confinement forhigh performance in the conventional non-cascade type-I diode. A furtheradvantage is that the type-I ICL should operate with high performance tolonger wavelengths than is possible for conventional non-cascade type-Idiodes, because the injectors of the cascade structure provide largervalence band offsets.

In summary, as described above, in accordance with the presentinvention, in both type-II and type-I ICLs, rebalancing theelectron/hole population ratio in the active quantum wells can beachieved by heavy doping of the electron injector, and such rebalancingimproves not only the threshold current density but also all of themajor laser performance figures of merit.

The carrier-rebalancing concept can also be applied to improve theperformance of interband cascade light-emitting diodes (ICLEDs). For theICLED, the figure of merit is the radiative efficiency at a givencurrent density rather than the optical gain and loss. Assumingoperation near room temperature, the current density is controlled byAuger processes with approximately equal contributions frommultielectron and multihole processes, as shown above. Using thenondegenerate model in which the radiative emission is proportional tothe product of the electron and hole sheet densities P_(s)N_(s), whilethe current density J is proportional to (P_(s)+N_(s))P_(s)N_(s), weobtain that the radiative efficiency η is proportional to1/(P_(s)+N_(s)).

Expressing the results in terms of the electron/hole population ratioR=N_(s)/P_(s) and eliminating the hole density in favor of the currentdensity, we obtain:

${\eta(J)} = {c\left\lbrack \frac{R}{{J\left( {1 + R} \right)}^{2}} \right\rbrack}^{1/3}$where c is a constant. We can find the maximum of the radiativeefficiency at a given current density by maximizing the quantity(η/c)³J, which yields R_(opt)=1. Therefore, carrier rebalancing bycarefully designing and doping the injector to obtain the same electronand hole sheet densities in the active wells is expected to produce themaximum ICLED efficiency, although within this simple model the benefitis less dramatic than for lasers. As the carrier densities approach andenter degeneracy, the radiative rate will saturate when R<<1 or R>>1,and the improvement due to rebalancing will actually be somewhat largerthan the nondegenerate model indicates. The unity value for the electronto hole population ratio can be achieved in the exemplary embodimentsfor both type-II and type-I lasers using sheet doping densities of1-2×10¹² cm⁻².

Although particular embodiments, aspects, and features have beendescribed and illustrated, it should be noted that the inventiondescribed herein is not limited to only those embodiments, aspects, andfeatures, and it should be readily appreciated that modifications may bemade by persons skilled in the art. For example, the precise dopingprofile (that is, the number of doped wells and their width) can bechanged while conserving the main principle of this invention, which tooptimize the charge balance in the active quantum wells by assuring thatat threshold at least as many electrons as holes are present. Theadvantages of the invention are expected to apply to interband cascadelasers emitting at any wavelength, although the optimal doping level anddoping spatial distribution may vary somewhat with wavelength. Most ofthe variations for type-II ICLs that are described here and in earlierpatent applications are also applicable to the type-I ICLs.

The present application contemplates any and all modifications withinthe spirit and scope of the underlying invention described and claimedherein, and all such embodiments are within the scope and spirit of thepresent disclosure.

What is claimed is:
 1. A type-II interband cascade gain medium,comprising: an active gain region having a plurality of cascadingstages, each of the cascading stages including type-II active gainquantum wells; a hole injector region adjacent the active gain quantumwells on a first side thereof, the hole injector region comprising atleast one hole quantum well and being configured to inject holes intothe active gain quantum wells; and an electron injector region adjacentthe active gain quantum wells on a second side thereof opposite thefirst side, the electron injector region comprising a plurality ofelectron quantum wells and being configured to inject electrons into theactive gain quantum wells, a hole injector of a first stage beingadjacent to an electron injector of a second stage, with a semimetallicinterface being present therebetween; wherein at least some of theplurality of the electron quantum wells in the electron injector regionare n-doped to a net sheet doping density of about 1.5 to about 7×10¹²cm⁻²; and wherein a ratio of electrons to holes in the active quantumwells is increased as a result of the n-doping in the electron injectorregion.
 2. The interband cascade gain medium according to claim 1,wherein the electron injector region is n-doped to a net sheet dopingdensity of about 1.5 to about 2.5×10¹² cm⁻².
 3. The interband cascadegain medium according to claim 1, wherein the electron injector issufficiently n-doped to achieve an electron/hole population ratio in theactive quantum wells of between about 0.7 and about
 2. 4. The interbandcascade gain medium according to claim 1, wherein the n-doped electronquantum wells comprise electron quantum wells in about the middle of theelectron injector.
 5. The interband cascade gain medium according toclaim 1, wherein the n-doped electron quantum wells comprise electronquantum wells directly adjacent to the active quantum wells or with atmost one undoped injector quantum well separating the doped injectorquantum wells from the active quantum wells.
 6. The interband cascadegain medium according to claim 1, wherein the n-doped electron quantumwells comprise electron quantum wells separated from the active quantumwells by at most one undoped injector quantum well.
 7. The interbandcascade gain medium according to claim 1, wherein the electron injectorcomprises six InAs electron quantum wells having a total injectorthickness of 180-200 Å, a thickness of an InAs electron quantum welladjacent to the semimetallic interface being about 30 Å; wherein theelectron injector is sufficiently doped to achieve a net sheet dopingdensity in the range 1.5-2.2×10¹² cm⁻²; and wherein the gain medium isconfigured to produce a laser emission having a wavelength in the rangeλ=3.0-3.8 μm.
 8. The interband cascade gain medium according to claim 1,wherein the electron injector comprises five InAs electron quantum wellshaving a total injector thickness of 180-200 Å, a thickness of an InAselectron quantum well adjacent to the semimetallic interface being about35 Å; wherein the electron injector is sufficiently doped to achieve anelectron sheet density in the range 2.0-2.5×10¹² cm⁻²; and wherein thegain medium is configured to produce a laser emission having awavelength in the range λ=5-6 μm.
 9. The interband cascade gain mediumaccording to claim 1, wherein the electron injector has a total injectorthickness of about 220 Å, a thickness of an electron injector quantumwell adjacent to the semimetallic interface being about 42 Å; whereinthe electron injector is n-doped to a total sheet doping density in therange 4-6×10¹² cm⁻², the doping being shifted towards the active quantumwells; and wherein the gain medium is configured to produce a laseremission having a wavelength in the range λ=3.0-3.8 μm.
 10. Theinterband cascade gain medium according to claim 1, wherein the electroninjector has a total injector thickness of about 180-190 Å, a thicknessof an electron injector well adjacent to the semimetallic interfacebeing about 42 Å; wherein the electron injector is n-doped to a totalsheet density in the range 2-5×10¹² cm⁻², the doping being shiftedtowards the active quantum wells; and wherein the gain medium isconfigured to produce a laser emission having a wavelength λ=5-6 μm. 11.A type-I interband cascade gain medium, comprising: an active gainregion having a plurality of cascading stages, each of the cascadingstages including a type-I active gain quantum well; a hole injectorregion adjacent the active gain quantum well on a first side thereof,the hole injector region comprising one or more hole quantum wells andbeing configured to inject holes into the active gain quantum well; andan electron injector region adjacent the active gain quantum well on asecond side thereof opposite the first side, the electron injectorregion comprising a plurality of electron quantum wells and beingconfigured to inject electrons into the active gain quantum well, a holeinjector of a first stage being adjacent to an electron injector of asecond stage, with a semimetallic interface being present therebetween;wherein at least some of the plurality of the electron quantum wells inthe electron injector region are n-doped to a sheet doping density ofabout 2 to about 7×10¹² cm⁻²; and wherein a ratio of electrons to holesin the active quantum well is increased as a result of the n-doping inthe electron injector region.
 12. The interband cascade gain mediumaccording to claim 11, wherein the electron injector has a totalinjector thickness of about 200-220 Å, a thickness of an electroninjector well adjacent to the semimetallic interface being about 45 Å;the electron injector region is n-doped to a sheet doping density ofabout 2 to about 3×10¹² cm⁻²; and wherein the gain medium is configuredto produce a laser emission having a wavelength λ=3-3.8 μm.
 13. Theinterband cascade gain medium according to claim 11, wherein theelectron injector is sufficiently n-doped to achieve an electron/holepopulation ratio in the active quantum wells is between 0.7 and
 2. 14.The interband cascade gain medium according to claim 11, wherein then-doped electron quantum wells comprise electron quantum wells in aboutthe middle of the electron injector.
 15. The interband cascade gainmedium according to claim 11, wherein the n-doped electron quantum wellscomprise electron quantum wells directly adjacent to the active quantumwells.
 16. The interband cascade gain medium according to claim 11,wherein the n-doped electron quantum wells comprise electron quantumwells with at most one undoped injector quantum well separating thedoped injector quantum wells from the active quantum well.
 17. Theinterband cascade gain medium according to claim 11, wherein theelectron injector includes three InAs electron quantum wells havingthicknesses of about 36, 30, and 26 Å, respectively, at the center ofthe electron injector region, the n-doping being concentrated in saidInAs electron quantum wells and the electron injector having a totalsheet doping density in the range about 2 to about 3×10¹² cm⁻².